What happens to fish that swim in waters tainted by traces of drugs that people take? When it's an antianxiety drug, they become hyper, antisocial and aggressive, a study found. They even get the munchies.

The findings, published online Thursday in the journal Science, add to the mounting evidence that minuscule amounts of medicines in rivers and streams can alter the biology and behavior of fish and other marine animals.

"I think people are starting to understand that pharmaceuticals are environmental contaminants," said Dana Kolpin, a researcher for the U.S. Geological Survey who is familiar with the study.

The Swedish researchers who did the study suspect the little drugged fish could become easier targets for bigger fish because they are more likely to venture alone into unfamiliar places.

"We know that in a predator-prey relation, increased boldness and activity combined with decreased sociality ... means you're going to be somebody's lunch quite soon," said Gregory Moller, a toxicologist at the University of Idaho and Washington State University.

Researchers around the world have been taking a close look at the effects of pharmaceuticals in extremely low concentrations, measured in parts per billion. Such drugs have turned up in waterways in the U.S., Europe and elsewhere over the past decade.

They come mostly from humans and farm animals; the drugs pass through their bodies in unmetabolized form. These drug traces are then piped to water treatment plants, which are not designed to remove them from the cleaned water that flows back into streams and rivers.

The research team at Sweden's Umea University used minute concentrations of 2 parts per billion of the antianxiety drug oxazepam, similar to concentrations found in real waters. The drug belongs to a widely used class of medicines known as benzodiazepines that includes Valium and Librium.

The team put young wild European perch into an aquarium, exposed them to these highly diluted drugs and then carefully measured feeding, schooling, movement and hiding behavior. They found that drug-exposed fish moved more, fed more aggressively, hid less and tended to school less than unexposed fish.

It is not clear exactly how long-term drug exposure, beyond the seven days in this study, would affect real fish in real rivers and streams.